Toby Harnden was the Daily Telegraph's US Editor, based in Washington DC, from 2006 to 2011. Click here for Toby's website. Follow him on Twitter here @tobyharnden and on Facebook here. He is the author of the bestselling book Dead Men Risen: The Welsh Guards and the Defining Story Britain's War in Afghanistan.

You were right, Mr Blair

Tony Blair was given a pretty rough ride by the British press in the Rose Garden today. President Bush accused us of trying to "dance on his political grave". But whatever one thinks about Iraq, there has been one shining achievement that cannot be taken away from the Prime Minister – peace in Northern Ireland.

Paisley and McGuiness, laughing together last week

I never thought it was possible. So at the British Embassy this evening I shook Mr Blair's hand and told him: "You were right about Northern Ireland and I was wrong." As one who supported the Iraq invasion, I added: "Let's hope we were both right about Iraq." The Prime Minister smiled and said: "Spero Tanto."

My wife tells me that Mr Blair, who holidays in Tuscany, was speaking in Italian and saying, roughly translated, "I very much hope so." One diplomat there thought the PM might have been speaking in Latin. I am afraid my lack of a classical education lets me down here.

But back to Northern Ireland, where I spent three and a half years as the Telegraph's correspondent from 1996 to 1999. Then, I was a dyed-in-the-wool sceptic about the peace process, believing the IRA was not serious about wanting to end the conflict and that a return to violence was almost inevitable.

Alastair Campbell, then Mr Blair's official spokesman, described my work as "vomit" and condemned me for "doom-mongering". Well, it has taken nine years since the Good Friday agreement for a final peace settlement to have been achieved. There have been many difficulties along the way and in 1998 a certain degree if scepticism was healthy.

But I, like many others, never imagined I would see the day when Ian Paisley and Martin McGuinness joined together in government, laughing and joking with each other. The word historic was bandied around far too often and far too prematurely in the 1990s. But historic it was when Paisley and McGuinness put the past behind them.

Mr Blair believed it could happen and he perservered. He was "dogged", to use one of the compliments Mr Bush had showered him with in the morning as we sat sweltering in the Rose Garden watching the end of an extraordinary political relationship.

No one likes to admit they were wrong, least of all journalists. But on this occasion I was glad to admit it. If I'd been right, no doubt many people enjoying happy, productive lives in Northern Ireland today would be in their graves.

The PM was at the Embassy for the final event of his farewell visit to Washington on the first leg of what is becoming known as the "legacy tour".

He had decided to hold a reception for all those who had helped solve, for the time being at least, the Irish Question

Mr Blair was relaxed and at ease. "One of the strangest things that ever happened to me," he said, "was sitting on the sofa in the Irish parliament with Ian Paisley on one side and Martin McGuinness on the other with the two of them getting on in a way that was quite unnerving, to be absolutely honest."

Northern Ireland, he suggested, could be a "metaphor for what political optimism can do". The "whole place has lifted off in this amazing way and it's a wonderful thing to see…it's a tremendous thing".

After the bashing Mr Blair had received earlier (though he acquitted himself well, giving a passionate and heartfelt defence not just of his own actions but of Britain's alliance with the US – check out the transcript here, he had every right to be so ebullient about his part in that achievement.

Condoleezza Rice was there to pay tribute to Mr Blair for leading "this great endeavour" in Northern Ireland. It showed, she said, what can be achieved "when great democracies work together".

She then made an interesting point about how we view historic events before and after they happen: "At the time, it seems impossible…to future generations it will always seem just inevitable." Peace in Northern Ireland was far from inevitable – and that makes the achievement of securing it all the greater.

In what seemed to be a reference to Iraq, Dr Rice said that it was "good to have an example of people overcoming their differences" in a place where "differences were a licence to kill".

Senator Ted Kennedy began to choke up as he recounted how he felt when a final agreement was reached and there were tears in people's eyes as they remembered "the loss of life and the violence that had taken place".

Congressman Richard Neal said: "What Gladstone tried, what Lloyd George tried, what Churchill tried, Tony Blair succeeded at…like Sisyphus, you remained committed to rolling the boulder up the hill."

Senator John Kerry was there – monopolising Mr Blair for 10 minutes and handing him a copy of his book, which the ambassador's wife quickly relieved him of – as were people like Niall O'Dowd of the "Irish Voice", John Mackey of the House International Relations Committee, Congressman Peter King and Cardinal McCarrick.

Of course, there can be no neat read across from Northern Ireland to the Middle East. There isn't much prospect of al-Qa'eda sitting down at the negotiating table, as the IRA eventually did, to hammer out a political settlement. By labelling the two groups "terrorist", we cannot even begin to bridge the chasm between their respective methods and aspirations.

But perhaps the Northern Ireland experience does show that when there seems to be no way out and that peace cannot be achieved, change is indeed possible if politicians show the courage not to admit defeat.

Will Tony Blair and George W. Bush be vindicated by history? Is some kind of peace in Iraq possible? Spero tanto indeed.